This invention relates to the recovery of humic acids, particularly from oxidised coal.
Various processes are known for producing a solid oxidised coal containing humic acids. One such process involves mixing coal with an aqueous medium to produce a slurry having a pH in the range 4 to 9, reacting the slurry with a gaseous oxidant selected from oxygen, air and mixtures thereof under conditions of temperature and pressure, and for a time, sufficient to cause the oxidation of the coal thereby producing oxidised coal containing humic acids, and separating the oxidised coal containing humic acids from the aqueous medium. This process is described in South African Patent Application No. 88/4770.
Another process is that described in South African Patent No. 87/9232 which involves oxidation of coal in the dry phase with oxygen or mixtures of oxygen and nitrogen in a fluidised bed reactor. The coal with a particle size in the range 100 microns to 3 mm is heated to a temperature of 150.degree. to 300.degree. C. under a partial pressure of oxygen for a time of 30 to 600 minutes.
The traditional way of extracting humic acids from oxidised coal is to slurry sodium hydroxide with the oxidised coal in water in equal amounts to give a concentration of 5 to 10 percent (m/v). For example, 20 grams of oxidised coal and 20 grams of sodium hydroxide are slurried in 300 ml of water.
The slurry is refluxed for 5 hours after which it is worked up to obtain humic acid according to the following scheme: ##STR1##